Multifunctional potentiometric gas sensor array with an integrated temperature control and temperature sensors

ABSTRACT

Embodiments of the subject invention relate to a gas sensor and method for sensing one or more gases. An embodiment incorporates an array of sensing electrodes maintained at similar or different temperatures, such that the sensitivity and species selectivity of the device can be fine tuned between different pairs of sensing electrodes. A specific embodiment pertains to a gas sensor array for monitoring combustion exhausts and/or chemical reaction byproducts. An embodiment of the subject device related to this invention operates at high temperatures and can withstand harsh chemical environments. Embodiments of the device are made on a single substrate. The devices can also be made on individual substrates and monitored individually as if they were part of an array on a single substrate. The device can incorporate sensing electrodes in the same environment, which allows the electrodes to be coplanar and, thus, keep manufacturing costs low. Embodiments of the device can provide improvements to sensitivity, selectivity, and signal interference via surface temperature control.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of U.S. application Ser. No.12/682,365, filed May 25, 2010, which is the U.S. National StageApplication of International Patent Application No. PCT/US2008/079416,filed Oct. 9, 2008, which claims the benefit of U.S. ProvisionalApplication Ser. No. 60/978,696, filed Oct. 9, 2007, all of which arehereby incorporated by reference herein in their entirety, including anyfigures, tables, or drawings.

The subject invention was made with government support under a researchproject supported by the Department of Energy, Contract Nos.DE-FG26-02NT41533 and DE-FC26-03NT41614. The government has certainrights to this invention.

BACKGROUND OF INVENTION

Potentiometric gas sensors based on measuring the potential differencebetween a semiconducting metal oxide and a noble metal pseudo-referenceelectrode in the same gas environment offer highly selective devicesthat are easily manufactured and can withstand harsh environmentswithout degrading performance. Furthermore, they are insensitive tolarge swings in O₂ concentration, such as those that occur in acombustion exhaust. Such solid-state potentiometric gas sensors showgreat promise for detecting pollutants such as NO_(x), CO, andhydrocarbons from ppb to ppm level concentrations for exhaustmonitoring. They also may be used in other applications such as in thebiomedical field for breath analysis.

Potentiometric gas sensors have an output voltage signal that can bemeasured in many different ways and can be used to determine individualgas concentration(s) in a gas mixture or that of a varying concentrationof single species in the absence of other gases. The voltage differencebetween two electrodes, which make up an “electrode-pair”, can bemonitored as the potential at one or each electrode changes.

Potentiometric gas sensors are utilized by measuring the output voltagesignal that can be used to determine individual gas concentration(s) ina gas mixture or that of a varying concentration of single species inthe absence of other gases.

Solid-state potentiometric gas sensors with semiconducting metal oxideelectrodes (such as p-type La₂CuO₄ (LCO)) have shown much promise forthe monitoring of pollutant gas (such as NO_(x)) levels in combustionexhaust. They are sensitive to ppm levels of NO_(x) and CO and have fastresponse times. Additionally, they are insensitive to vacillating O₂ andCO₂ concentrations. However, the selectivity and cross-sensitivity ofthese sensors is currently inadequate for commercial application. Aprime example of this is the inability to discriminate between NO andNO₂ (the primary components of NO_(x)). It is often important to knowthe concentration of each of these individual gases; however, mostNO_(x) sensors cannot determine which of these species is present ordetermine their absolute concentration in mixed gas streams. In fact,poor selectivity hinders most solid-state pollutant sensors. Currentlyavailable devices for monitoring combustion exhausts and/or reactionbyproducts are limited in several ways. Current devices detect only onegas species or detect multiple species only by utilizing expensiveelectronics to extrapolate the gas concentrations from the measurementor to take the measurement.

Current devices can require an air reference, which complicates thedesign, and/or have complicated manufacturing steps that increase cost.

A reference electrode is typically used to compare the changing EMF of asensing electrode to an EMF that does not change (i.e., a referencestate). A pseudo-reference is an electrode which can be used to compareall other sensing electrodes in a single gas environment. However, thepseudo-reference has an EMF that changes at the same time that thesensing electrodes are changing. Accordingly, a pseudo-reference doesnot actually represent a true reference state.

BRIEF SUMMARY

Embodiments of the subject invention relate to a gas sensor and methodfor sensing on or more gases specific embodiments pertain to apotentiometric gas sensor and method for sensing on or more gases.Additional embodiments are directed to amperometric and/or impedimetricgas sensors and method for sensing one or more gases. An embodimentincorporates an array of sensing electrodes maintained at similar ordifferent temperatures, such that the sensitivity and speciesselectivity of the device can be fine tuned between different pairs ofsensing electrodes. A specific embodiment pertains to a gas sensor arrayfor monitoring combustion exhausts and/or reaction byproducts. Anembodiment of the subject device related to this invention operates athigh temperatures and can withstand harsh chemical environments.

Embodiments of the device are made on a single substrate. In otherembodiments, several different single electrode-pair devices can beproduced on separate substrates. The device can incorporate sensingelectrodes in the same environment, which allows the electrodes to becoplanar and, thus, keep manufacturing costs low. Embodiments of thedevice can provide improvements to sensitivity, selectivity, and signalinterference via surface temperature control.

Embodiments of the subject device can have a single pseudo-reference.Other embodiments can use all of the electrodes as pseudo-referenceswith respect to each other. The electrodes can be viewed as making up“electrode-pairs,” which can be measured as a potential differencesignal. The voltage difference between two electrodes (which make up an“electrode-pair”) is measured as the potential at one or each electrodechanges. Embodiments can also have as a reference another fixed voltage,such as that provided by a battery, other power source or the chassis ofan automobile.

Sensing electrodes can be metals (e.g., Platinum or Gold),semiconductors (e.g. semiconducting oxides such as La₂CuO₄ or WO₃), orany other material showing sensitivity to a single or multiple gasspecies. Typically, a given sensing electrode material will have varyingsensitivity (i.e., changes in EMF) and selectivity to one or manydifferent gas species This depends on the temperature of each electrodeand the difference in temperature between electrodes making up anelectrode-pair. This will also depend on the concentration and chemicalproperties of the particular species interacting with the material. Thedegree of sensitivity and/or selectivity that changes depends on thematerial and its properties, gas species present, and the temperature.Since each electrode may be part of one or more “electrode-pairs,” thenumber of measurable signals can be larger than the actual number ofsensing electrodes.

The presence of more signals than actual number of electrodes can be anadvantage in a device. Typically, with a greater number of signals thepattern recognition of multiple gas species becomes easier. The voltageresponse of electrode-pairs can be measured over a variety of knownconditions, including exposures to one or more gas speciesconcentrations, and these measurements can be used to interpret themeasurements taken during exposures to unknown gas speciesconcentrations (i.e., the sensor may be calibrated). Therefore, theresult of having more signals than total number of electrodes means thata device will require fewer electrodes for the same or betterselectivity. This, in turn, means reduced costs and the possibility ofsmaller devices.

The design of the sensor array can include, either as individual devicesor together in a single device, two different “electrode-pair” schemes.One scheme uses multiple materials at the same time, which may be keptat the same and/or different temperature (using heating or coolingmethods). A device may also include multiple electrodes of the samematerial that are maintained at one or more different temperatures.

The electrodes of the same material may be kept at the same temperatureif other features of the electrode, such as microstructure (e.g., grainsize or surface roughness), size, shape, or thickness, are different.Again, the gas sensor array may utilize one of these schemes or both ofthese schemes in a single device (or multiple devices), depending on theapplication.

Any given sensing electrode material is typically sensitive to more thanone gas species. This sensitivity varies with temperature and gasspecies. Therefore, one can measure a signal from two electrodes of thesame material if they are modified in a way that alters the sensitivityof at least one of the electrodes making up an electrode-pair. Thesensitivity of a given electrode material can be modified by differencesin its microstructure, geometry, temperature, or other method whichchanges the local environment of the electrode to enhance or alterchemical (or electrochemical) reactions in a desired way. The samemodifications may be used to yield a measurable electrode-pair made upof dissimilar materials.

To be cost effective, the device(s) can be made on a single substrate.Furthermore, the device(s) can have sensing electrodes in the sameenvironment, which allows the electrodes to be coplanar (i.e., all onone side of the substrate) and, thus, avoiding complex designs whichmight increase manufacturing costs. The sensitivity and selectivity ofthese sensors varies with temperature. Therefore, the temperature ofsuch device(s) can be controlled and enabled to be modified quickly ifthe ambient temperature changes or if the electrode temperature changesfor any other reason.

In order to achieve a device that is able to monitor two or more gasspecies of interest, an array of sensing electrodes can be used. Thearray signals can then be entered into algorithms to determine theconcentrations of individual species. Pattern recognition can beimplemented to determine the concentrations of individual species. Byimproving selectivity, a device can have fewer sensing electrodes toeffectively detect the same species as a device with more signals butincreased cross-sensitivities. This can simplify the device and lowerthe power consumption and the cost of constructing the device.

Heaters can be utilized with the subject invention in order to controlthe temperature of one or more of the sensing electrodes. Such heaterscan use one or more heating elements through which current can be drivento create heat so as to alter the temperature of the sensing electrodes.The heating elements can use any conducting or resistive material (e.g.,Platinum) which has the thermal and chemical stability necessary to keepit (and its performance) from degrading with time and in a harshenvironment. The heating elements can act as resistors. The heat isproduced via Joule heating, or passing electrical current through theheating elements. The heat generated is proportional to the square ofthe current multiplied by time. Additional embodiments can use a coolingapparatus to lower the temperature of the sensing electrodes. A varietyof cooling techniques known to those skilled in the art can beincorporated into embodiments of the invention for this purpose.

Temperature control of embodiments of the subject devices can beaccomplished in a number of ways. Precise control of temperature withminimal fluctuations is useful to achieving stable sensor signals.Therefore, thermal modeling can provide a way to design the temperatureprofile for the device. This information can be used when determiningwhere to locate individual electrodes on the substrate of the array orhow the temperature profile will change in varying gas flow velocities.

Surface temperature measurements can be difficult. Knowledge of thetemperature of the sensing electrodes can enhance the deviceperformance. The resistance of some metals, semiconductors, or othermaterials will change with temperature in a way which can be predictedby various mathematical models. After the data is fit to a model,software can easily calculate the surface temperature during sensoroperation using the coefficients from the model and resistancemeasurements of the temperature sensor elements. In a specificembodiment, resistance measurements, or other temperature determiningtechnique, can be applied to the sensing electrode, for example beforeor after the gas sensing measurement, in order to provide a value forthe temperature of the sensing electrode. Additionally, temperaturesensors that utilize changes in voltage (e.g., thermocouple) orcapacitance as a detection method may also be integrated into thedevice.

Heating elements can be used not only to heat another object but alsosimultaneously as a temperature sensor. If the resistance of the heatercan accurately be determined (e.g., using a four-wire method), then thetemperature of the heating element (and thus that of the sensingelectrode) can be calculated. Resistance typically increases as currentis supplied to the heater because of Joule heating. This does notgreatly affect the voltage or current measurements. That is to say, themeasurements represent the actual current in the circuit and voltagedrop across the heater. Therefore the calculated resistance, and hencetemperature, of the heater represents the real value.

The heating element shape can be designed to ensure that temperature ofany given sensing electrode is uniform, or, if desired, designed so thatthe temperature is purposefully nonuniform. The heating elements may beC-, spiral-, serpentine-shaped, or any other useful pattern to achievethe desired thermal distribution throughout the device. The heatingelements can be controlled either by an applied voltage or current. Themethod that is chosen depends on the application. For example in anautomobile, the likely power source will be the automobile's battery.The heating elements could, therefore, be voltage controlled.

A single heating element (or temperature sensor, or cooling element) ormultiple heating elements (or temperature sensors, or cooling elements)may be used to control the temperature of any given sensingelectrode(s).

Heating elements (or temperature sensors, or cooling elements) may beunderneath (and appropriately aligned with) an individual or multiplesensing electrode(s), separated from the sensing electrode(s) and solidelectrolyte by one or more thermally insulating or thermally conductinglayers.

The heating elements (or temperature sensors, or cooling elements) maybe separated from each other by thermally insulating or thermallyconducting layers, by the geometry of the substrate or other layers, orby empty spaces between them.

The heating elements (or temperature sensors, or cooling elements) maybe suspended in cavities for thermal isolation from other regions of thedevice.

The heating elements (or temperature sensors, or cooling elements) mayalso be completely covered by thermally insulating or thermallyconducting layers (i.e., embedded in the device) and may exist in any ofthe device layers.

Platinum may be selected for the fabrication of heating elements,temperature sensors, and/or cooling elements. Platinum is an industrystandard for high-temperature resistance-temperature-devices (RTD) andas heating elements in gas sensors because of durability and chemicaland thermal stability. However, other materials may be used as heatersin such devices. Also, other materials may be used for the temperaturesensors or cooling elements.

Also, with the incorporation of temperature control into such devices itmay be possible to reverse electrode “poisoning” or other phenomena thatkeep the device from responding in a repeatable way for exposure to agiven gas(es) and concentration(s) which results in changes in sensorperformance over time or complete failure of the device.

Embodiments of the invention can improve selectivity of more than onegas species and/or can improve the sensitivity to more than one gasspecies. A single device with an array of electrode-pairs can bothimprove sensitivity and selectivity.

The device shown in FIGS. 1 and 2 includes a sensor array withintegrated Platinum heater and temperature sensors that were fabricatedfor small size and low power-consumption. The array includes two(semiconducting) La₂CuO₄ (LCO) electrodes 1, 3 and a Platinum (Pt)reference electrode 2 all on the same side of rectangular, tape-castYttria-Stabilized-Zirconia (YSZ) substrate 4. In alternativeembodiments, all three electrodes can be one material, such as LCO, oreach electrode can be a different material. Platinum resistor elementsare used as heaters 5 and/or temperature sensors 5, 6, 7 to control andmonitor the temperature of the sensing electrodes. Finite ElementModeling was used to predict temperature profiles within the array. Thearray was then designed to keep LCO electrode 1 hotter with respect tothe other two electrodes. The results from this device demonstrated thata gas sensor array with sensing electrodes kept at differenttemperatures can yield a device capable of selectively determining NOand NO₂ concentrations. The individual concentrations of these gases canbe calculated during operation. Different sensing electrode materialsand/or different temperature differentials between sensing electrodescan be used for detection of other gases and/or determination of theconcentrations of other gases.

Referring to FIGS. 7-8, a different gas sensor array based on YSZ 12includes two LCO sensing electrodes and two Platinum referenceelectrodes. The inner LCO 9 and Pt 10 electrodes are heated, while theouter LCO 11 and Pt 8 electrodes remain near the ambient temperature. Ptelements 14 and 15 are used to heat and measure temperature, while 13and 16 are used only to measure temperature. This device offers theability to measure the potential difference between multiple pairs ofelectrodes. In further embodiments, the heating elements and/ortemperature sensing elements can be located on the same side of thesubstrate as the sensing electrodes or detached from the substrate.

From the trends with changes in specific electrode temperatures, theslopes of the plots in FIGS. 9-10 (and similar sensor response plots forthe other electrode-pairs), which represent the sensitivity (mV changein signal per decade change in gas concentration) were used to makesensitivity plots in FIGS. 11-16. Each line represents a differentheater setpoint, which in turn represents a separate temperaturedifference (|dT|) between the electrodes as shown. This was repeated foreach of the six signals from the four sensing electrodes. In the trendplots, the case where |dT| is zero represents measurements when theheaters were not being operated.

FIG. 17A shows a contour plot for temperature variation in the sensorarray of FIGS. 7-8 during operation. Each contour in the plot representsa given temperature within the device. A temperature profile through themiddle of the device can be seen in FIG. 17B. Note that the sensor arrayof FIGS. 7-8 was made by hand and the results are therefore notnecessarily ideal. Therefore, each of the electrodes, even when made ofthe same material, was slightly different from each other. When theelectrode-pair is of the same material and the heater is not beingoperated, the sensitivity should be zero. However, as indicated in theplots the sensitivity is in fact nonzero.

Also note that the plots of FIGS. 9-16 are labeled to show therespective electrode-pairs, which make up six unique signals. In theplots, electrodes 8, 9, 10, and 11 from FIGS. 7-8 are designated asPt(1), LCO(2), Pt(3), and LCO(4), respectively. At a certain setpoint,the unheated electrodes slightly began to increase in temperature due tothe specific design of this array. This can be corrected very easilywith minor changes in the design such as moving the unheated electrodesfurther away from the heated electrodes, or creating a thermalinsulation barrier. The device can be improved with changes in theheater design and layout of the electrodes with respect to the heatersand to each other. Also, the heaters can be arranged differently withrespect to each other. Thermal modeling helps determine what to expectin device performance with respect to temperature uniformity.

Referring to FIGS. 11 and 12, showing the signals from the LCO(4)-LCO(2)and Pt(3)-Pt(1) electrode-pairs, the sensitivity of the electrode pairsis changed as the temperature difference between them increases. ForLCO(4)-LCO(2), the NO sensitivity significantly increases as thetemperature of the heated electrode, LCO(2), rises. In fact there isalmost an increase of ten times the initial sensitivity when notemperature difference exists between the electrodes. As the heatersetpoints increase, the NO₂ sensitivity decreases to almost zero. Thereis a slight increase in sensitivity at later setpoints, but at leastover a small range of the setpoints this electrode-pair is insensitiveto NO₂. Therefore this electrode-pair shows sensitivity only to NO andshould be NO selective. The signals from Pt(3)-Pt(1), furtherdemonstrate that by changing the temperature of individual electrodes ofthe same material, the sensitivity can be changed.

Referring to FIGS. 13 and 14, showing the signals from the LCO(2)-Pt(3)and LCO(4)-Pt(1) electrode-pairs, the sensitivity of the electrode pairsis changed as the temperature difference between them increases. ForLCO(2)-Pt(3), the electrode-pair effectively has become insensitive toNO. However, the NO₂ sensitivity becomes more positive and changes froma negative response to a positive response as the temperature differencebetween the electrodes increases. Therefore, this electrode-pair isselective to NO₂. For LCO(4)-Pt(1), the NO sensitivity remains nearlyfixed at the level where the signal is without the heater in operation.This demonstrates that by changing the temperature of individualelectrodes, the sensitivity can be changed for electrodes of differentmaterials.

Referring to FIGS. 15 and 16, showing the signals from the LCO(4)-Pt(3)and LCO(2)-Pt(1) electrode-pairs, the sensitivity of the electrode pairsis changed as the temperature difference between them increases. ForLCO(4)-Pt(3) the sensitivity to NO nearly doubles with respect to thecondition without a difference in temperature between the electrodes.Also, the sensitivity to NO₂ becomes more positive and changes from anegative response to a positive response as the temperature differencebetween the electrodes increases. This indicates that at sometemperature difference between the two electrodes, the NO₂ sensitivityshould go to zero. For LCO(2)-Pt(1), the NO sensitivity becomesincreasingly negative as the temperature difference between theelectrodes is increased. This shows that large changes in sensitivity toboth NO and NO₂ are possible by having different temperatures ofelectrodes making up an electrode-pair.

FIGS. 18 through 21 demonstrate a variety of additional sensorembodiments that are possible. FIG. 18A represents a cross-section of adevice similar to that shown in FIGS. 1 and 2 and FIGS. 7 and 8. In thisembodiment, electrolyte layer 17 is still coupled with sensingelectrodes 18 (which can be the same or different from each other).However, Pt elements 19 (used as heaters and/or temperature sensors),exist on top of support material 20. The support may be an electricinsulator or electrolyte, which may be the same or different fromelectrolyte layer 17. The electrolyte 17 (and attached sensingelectrodes 18) cover the Pt elements 19 and also sit atop the support20. The embodiment shown in FIG. 18B is similar to that shown in FIG.18A, with sensing electrodes 21 still being coupled to an electrolytelayer 22 on top of support 23. The main difference is that the Ptelements 24 are now embedded in the support 23. In FIG. 18C, the deviceincorporates a support material 25 with sensing electrodes 26 andelectrolyte 27 on top. The electrolyte layer 27 is in contact withsupport 25. Pt elements 28 exist on the backside of support 25. FIG. 18Dincorporates one electrolyte layer 29 for one (or more than one)electrode-pair made up of (same or different) sensing electrodes 30.Another electrolyte layer 31, also with sensing electrodes 30, existsseparately from electrolyte 29. Both electrolyte layers 29 and 31 existon top of support 32. Backside Pt elements 33 exist on the support aswell. Multiple combinations of this arrangement are possible.

FIG. 19 shows a cross-section of an embodiment which has (same ordifferent) sensing electrodes 34 on one side of an electrolyte 35, whichhas Pt elements 36 embedded in side. On the other side of electrolyte36, are additional (same or different) sensing electrodes 37. Electrodepairs may be made up of any combination of sensing electrodes 34 and 37.Having sensing electrodes on opposite sides of the device results in aseparation of the local gaseous environment around each sensingelectrode, and in certain situations will result in a reduction ofcross-talk and improved selectivity.

FIG. 20 is a cross-section of an embodiment which has a hollow chamberin the middle of the device. In a fashion similar to that used in theembodiment of FIG. 19, this chamber acts to separate the localenvironment of the sensing electrodes and can be used to provide aseparate gas stream of known concentration as a reference. The deviceincorporates (same or different) sensing electrodes 38 on the outsideand (same or different) sensing electrodes 39 inside of the hollow spaceand attached to electrolyte 40. Pt elements 41 may exist on the inside(or outside) of the chamber, also attached to electrolyte 40. AdditionalPt elements for heating or temperature sensing may be arranged about thedevice in various locations.

FIGS. 21A and 21B show the top view of embodiments where the electrodearrangement relative to the substrate is different from that shown inFIGS. 1 and 2 and FIGS. 7 and 8. FIG. 21A shows an embodiment with (sameor different) sensing electrodes 42 atop an electrolyte and/orstructural support 43. Compared to other embodiments, the sensingelectrodes 42 are staggered and separated from each other on the top ofthe electrolyte (support) 43. FIG. 21 B shows an embodiment where thesensing electrodes 44 are oriented in a different manner with respect tothe electrolyte and/or support 45 and gas flow direction than theembodiment shown in FIG. 21A. Various arrangements and features such asthose shown in other embodiments may be used for Pt elements (used asheaters and/or temperatures sensors), other temperature sensors orcooling elements, with respect to these and other embodiments.

FIGS. 22 through 31 represent signals from the device in FIGS. 7 and 8,demonstrating that using a method and/or apparatus in accordance withembodiments of the invention, a sensor array can be made selective to aspecific gas species as the temperature of individual electrodes ischanged. Again, the difference in the temperature between electrodes andthe absolute temperature of each electrode is important for sensorperformance. FIGS. 22 through 25 show the NO_(x) gas mixture results forthe LCO(2)-Pt(1) signal, while the LCO(4)-LCO(2) signal is demonstratedin FIGS. 26 through 31.

FIG. 22 represents the LCO(2)-Pt(1) sensor response to NO₂ gas exposurefor gas mixture conditions of 0 ppm NO (solid lines) and 200 ppm NO(dashed lines). The x-axis of the plot has a log scale. The square,circle, and diamond symbols represent the conditions where 0, ˜13, and˜54 mW of total power were delivered to the Pt heating elementsresulting in greater temperature differences (dT) between electrodes. Ascan be seen, the slope of each set of lines, which represents thesensitivity (mV change in signal per decade change in gas concentration)to NO₂, increases with application of heater power. Furthermore, thesensitivity is mostly unaltered by addition of NO during NO₂ exposure.FIG. 23 shows the sensor response for NO gas exposure with 0 ppm NO₂(solid lines) and 200 ppm NO₂ (dashed lines). The x-axis of the plot hasa log scale. As seen in FIG. 23, the sensitivity to NO decreases as thepower to the heaters increases. Also marked in this figure for eachheater setpoint are the approximate shifts in NO sensitivity when 200ppm NO₂ is added to the gas mixture. The shifts are all negative asexpected when considering the larger (negative) response to NO₂ as shownin FIG. 22. As the heater power increases, the shift becomes moreuniform over the entire range of NO concentrations probed. At lowerheater power, the shift is more prominent for higher NO concentrations(i.e., the sensitivity decreases with addition of 200 ppm NO₂). Withoutthe use of the heaters, the shift in the sensor response is 0.18 to 1.3mV along the entire NO concentration range. For 13 mW of heater power,the shift is between 3.2 and 3.7 mW. At 54 mW, the heater power issufficient to reduce the NO response to such a degree that the curve ishorizontal. When 200 ppm NO₂ is introduced, the curve remains horizontalbut shifts to more negative values by 6.8 mV.

FIG. 24 shows the NO₂ sensor response with the same conditions as FIG.22 for 0 ppm NO. However, the x-axis has a linear scale and this plotincludes data points for the condition of 0 ppm NO₂. Also marked in FIG.24, are the difference in the measured voltage difference between the 0ppm NO₂ baseline and the 200 ppm NO₂ gas step. When the results for NO(0 ppm and 200 ppm NO₂) in FIG. 23 are compared to the changes involtage between the 0 ppm NO₂ baseline and 200 ppm NO₂ gas step (FIG.24), the improvements in NO₂ selectivity with increasing heater powerare clear. Without the use of the heaters, a change from 0 ppm to 200ppm NO₂ produces a change in voltage of 3.5 mV (FIG. 24), while there isa shift of 0.18 to 1.3 mV between these two conditions when NO is alsopresent in the gas mixture (FIG. 23).

This difference can be debilitating when trying to determine NO and/orNO₂ gas concentrations in a gas mixture because the actual voltagemeasured is different from that which is expected. When a small amountof power is delivered to the heaters (˜13 mW), the situation improvesslightly as evidenced when comparing the expected change in voltage of 5mV (FIG. 24) to the actual change of 3.2 to 3.7 mV seen when NO and NO₂are present (FIG. 23). As mentioned previously, a heater setpointdelivering 54 mW of power results in increased NO₂ sensitivity (FIG. 22)and a complete removal of NO sensitivity (horizontal curve in FIG. 23).Furthermore, the expected voltage change between conditions of 0 ppm and200 ppm NO₂ is 6.5 mV (FIG. 24). This is almost exactly the same as theshift (6.8 mV) during NO gas exposure when measurements are also made inthe presence of 0 ppm and 200 ppm NO₂. Now that gas mixtures of NO andNO₂ do not affect the expected voltage change to variations in NO₂, thegas sensor array can be used to accurately portray the realconcentration of NO₂ gas present in the gas mixture. Using the sameprinciples, the sensor array can be made to have improved selectivity toany gas, such as NO, NO₂, NH₃, CO, CO₂, and/or hydrocarbons.

FIG. 25 shows a plot of sensitivity versus total heater power for theLCO(2)-Pt(1) signal of the embodiment in FIGS. 7 and 8 with gas mixtureconditions of NO (0 and 200 ppm NO₂) and NO₂ (0 and 200 ppm NO), asindicated. The sensitivity to NO, with and without the presence of NO₂,decreases to 0 mV/decade ppm NO as the heater power increases. As thishappens, there is also a decrease in the change in sensitivity seen when200 ppm NO₂ is introduced into the gas mixture. The sensitivity to NO,with and without the presence of NO₂, decreases to 0 mV/decade ppm NO asthe heater power increases. As the heater power increases, thesensitivity to NO₂, with and without the presence of NO, almostincreases by a factor of 2. The sensitivity to NO₂ with 0 ppm and 200ppm NO, remains mostly unchanged over the same range of heater power.Moreover, by operating this electrode pair at the maximum dT (obtainedwith 54 mW of heater power) a sensor is obtained that has both highersensitivity and selectivity to NO₂, since the cross sensitivity to NO isremoved (becomes zero or negative). When considering these changes insensitivity and the voltage shifts observed with exposure of gasmixtures of NO and NO₂ as mentioned earlier, it is clear that theoverall sensor array performance can be enhanced using embodiments ofthe subject method.

FIGS. 26 through 28 show how the LCO(4)-LCO(2) electrode-pair, of theembodiment of FIGS. 7 and 8, can be used to detect total NO_(x)concentrations when NO and NO₂ exist in a gas mixture together. TheLCO(4)-LCO(2) response to NO₂ gas exposure for gas mixture conditions of0 ppm NO (solid lines) and 200 ppm NO (dashed lines) is shown in FIG. 26for total heater power of 0, 13, and 54 mW as indicated. For the sametotal heater power, FIG. 27 shows the response to NO gas exposure forgas mixture conditions of 0 ppm NO₂ (solid lines) and 200 ppm NO₂(dashed lines). Referring to FIGS. 26 and 27, the response to NO₂ (0 and200 ppm NO) gas mixtures always shows a positive response. The same istrue for NO (0 and 200 ppm NO₂) gas mixtures and except for the casewhen the heaters are not used (0 mW total heater power), where there isessentially no sensitivity to NO. Furthermore, the shift in theLCO(4)-LCO(2) signal is always positive when NO is introduced to NO₂ gassteps, as in FIG. 26, and when NO₂ is added to NO gas steps, as in FIG.27. When there is a shift in response for both the case in FIGS. 26 and27, the slope remains relatively unchanged, even at the higher totalheater power setting. This is shown in FIG. 28, which is a plot ofsensitivity (mV/decade ppm NO or NO₂) versus total heater power for NO(0 and 200 ppm NO₂) and NO₂ (0 and 200 ppm NO), as indicated. Also notein this figure, that the sensitivity to both NO and NO₂ increases withincreasing total heater power as the temperature of the LCO(2) electrodeincreases. A unique voltage difference is produced for each combinationof NO and NO₂ concentrations. This is demonstrated in FIGS. 29 through31, which show the sensor response versus total ppm NO_(x) in the gasmixture for 0 mW, 13 mW, and 54 mW respectively. In the case where theheaters are not used (FIG. 29), the LCO(4)-LCO(2) signal is insensitiveto NO but has sensitivity to NO₂. Therefore, under these conditions theLCO(4)-LCO(2) electrode-pair is selective to NO₂. However, as evidentfrom FIGS. 29 through 31, as the temperature of the heated LCO(2)electrode increases (i.e., when the heater power is applied), the totalNO_(x) measurement becomes possible as the signal begins to becomesensitive to NO, while remaining sensitive to NO₂. Comparing FIGS. 30and 31, as the heater power is increased further, the sensitivity to NOand NO₂ increases even more. Furthermore, there is overlap between thegas mixture measurements involving NO (0 and 200 ppm NO₂) and NO₂ (0 and200 ppm NO). For example, at a total NO_(x) concentration of 400 ppm(200 ppm NO and 200 ppm NO₂), the sensor response is exactly the sameregardless of whether the measurement was made in dynamic gas steps ofNO₂ with static NO concentration, or vice versa. In summary, by changingthe temperature of at least one sensing electrode with respect toanother, it becomes possible to measure the total NO_(x) in gas mixturesof NO and NO₂ even when using the same materials for each electrodemaking up the electrode-pair.

As demonstrated in FIGS. 22 through 31, the embodiment in FIGS. 7 and 8and similar sensor arrays have the capability of detecting theindividual concentrations of NO and NO₂. This is possible because theLCO(2)-Pt(1) electrode-pair can selectively detect NO₂ over NO in NO_(x)gas mixtures when the LCO(2) electrode is heated locally. Furthermore,the LCO(4)-LCO(2) electrode-pair, which is made up of two sensingelectrodes of the same material but different temperatures, is able todetect total NO_(x). The concentration of NO can be calculated bysubtracting the detected NO₂ concentration from the detected NO_(x)concentration. Though this method is indirect, it is possible that usingthe same method of locally heating individual electrodes making upelectrode-pairs of similar or different temperatures that anelectrode-pair(s) can provide selective detection of NO, NO₂, (or CO,CO₂, ammonia, and other gases) as demonstrated in FIGS. 9 through 16.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a specific embodiment of a device in accordance with thesubject invention.

FIG. 2 shows a cross-sectional view of the embodiment of FIG. 1.

FIG. 3 shows the sensor response vs. concentrations of NO₂ for thenon-heated LCO electrode vs. the non-heated platinum electrode takenfrom the three sensing electrodes.

FIG. 4 shows the sensor response vs. concentrations of NO₂ for theheated LCO electrode vs. the non-heated platinum electrode taken fromthe three sensing electrodes.

FIG. 5 shows the sensor response vs. concentrations of NO₂ for theheated LCO electrode vs. the non-heated platinum electrode taken fromthe three sensing electrodes.

FIG. 6 shows the sensor response vs. concentrations of NO₂ for thenon-heated LCO electrode vs. the non-heated platinum electrode takenfrom the three sensing electrodes.

FIG. 7 shows an additional embodiment of the subject invention.

FIG. 8 shows a cross-sectional view of the embodiment in FIG. 7.

FIG. 9 shows the sensor response vs. concentrations of NO for theelectrode pair having the non-heated LCO sensing electrode and theheated platinum sensing electrode, LCO(4)-Pt(3), showing the results forincreasing temperatures difference.

FIG. 10 shows the sensor response vs. concentrations of NO₂ for theelectrode pair having the non-heated LCO sensing electrode and theheated platinum sensing electrode, LCO(4)-Pt(3), showing the results forincreasing temperatures.

FIG. 11 shows the signal results from the LCO(4)-LCO(2) and Pt(3)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO.

FIG. 12 shows the signal results from the LCO(4)-LCO(2) and Pt(3)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO₂.

FIG. 13 shows the signal results from the LCO(2)-Pt(3) and LCO(4)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO.

FIG. 14 shows the signal results from the LCO(2)-Pt(3) and LCO(4)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO₂.

FIG. 15 shows the signal results from the LCO(4)-Pt(3) and LCO(2)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO.

FIG. 16 shows the signal results from the LCO(4)-Pt(3) and LCO(2)-Pt(1)electrode-pairs, of the embodiment in FIGS. 7 and 8, in response tochanges in gas concentration of NO₂.

FIG. 17A shows the temperature contour plot for embodiment shown inFIGS. 7 and 8.

FIG. 17B shows the temperature profile through the cross-section of FIG.17A.

FIG. 18A shows an embodiment with structural support and electrolytewith embedded heaters and sensing electrodes deposited on top, where amultitude of electrode pairs may exist.

FIG. 18B shows an embodiment with structural support with embeddedheaters and electrolyte and sensing electrodes deposited on top, where amultitude of electrode pairs may exist.

FIG. 18C shows an embodiment with structural support with backsideheaters and topside deposited electrolyte and sensing electrodes, wherea multitude of electrode pairs may exist.

FIG. 18D shows an embodiment with structural support with backsideheaters and separate electrolyte layers with sensing electrodes fordifferent electrode-pairs, where a multitude of electrode pairs andelectrolyte layers may exist.

FIG. 19 shows an embodiment with an electrolyte support with embeddedheaters, and sensing electrodes on opposite sides of the electrolyte,where a multitude of electrode pairs and electrolyte layers may exist.

FIG. 20 shows an embodiment with one or more chambers inside thestructural electrolyte, with heaters deposited on one side of thechamber, sensing electrodes are positioned on the other side, andadditional sensing electrodes positioned on the outside of thestructural electrolyte, where the chamber can be used for a referencegas.

FIG. 21A shows an embodiment with sensing electrodes staggered andseparated from each other on the top of the electrolyte and/orstructural support.

FIG. 21B shows an embodiment the sensing electrodes oriented in adifferent manner with respect to the gas flow direction.

FIG. 22 shows a (log scale) plot of the sensor response to NO₂ for theLCO(2)-Pt(1) electrode-pair of the embodiment in FIGS. 7 and 8, testedat a higher ambient temperature than for FIGS. 9 to 16, for severaldifferent instances of total heater power, where conditions testedinclude gas steps of NO₂ with 0 ppm NO and 200 ppm NO gas mixtures.

FIG. 23 shows a (log scale) plot of the sensor response to NO for theLCO(2)-Pt(1) electrode-pair of the embodiment in FIGS. 7 and 8, testedat a higher ambient temperature than for FIGS. 9 to 16, for severaldifferent instances of total heater power, where conditions testedinclude gas steps of NO with 0 ppm NO and 200 ppm NO₂ gas mixtures,where the shifts in response caused by introduction of NO₂ into the NOgas stream are also marked.

FIG. 24 shows a (linear scale) plot of FIG. 22, for the embodiment inFIGS. 7 and 8, with the voltage change from 0 to 200 ppm NO₂ marked foreach heater power condition.

FIG. 25 shows sensitivity versus total heater power for the LCO(2)-Pt(1)electrode-pair, for the embodiment in FIGS. 7 and 8, taken from FIGS. 22and 23.

FIG. 26 shows the NO response of the LCO(4)-LCO(2) electrode-pair of theembodiment in FIGS. 7 and 8, tested at a higher ambient temperature thanfor FIGS. 9 to 16, for several different instances of total heaterpower.

FIG. 27 shows the NO₂ response of the LCO(4)-LCO(2) electrode-pair ofthe embodiment in FIGS. 7 and 8, tested at a higher ambient temperaturethan for FIGS. 9 to 16, for several different instances of total heaterpower.

FIG. 28 shows the sensitivity versus total heater power for theLCO(4)-LCO(2) electrode-pair, for the embodiment in FIGS. 7 and 8, takenfrom FIGS. 26 and 27.

FIG. 29 demonstrates the total NO_(x) sensing capability of theLCO(4)-LCO(2) electrode-pair, for the embodiment in FIGS. 7 and 8,without the use of the heaters (i.e., total heater power is 0 mW).

FIG. 30 demonstrates the total NO sensing capability of theLCO(4)-LCO(2) electrode-pair, for the embodiment in FIGS. 7 and 8, for atotal heater power of 13 mW.

FIG. 31 demonstrates the total NO_(x) sensing capability of theLCO(4)-LCO(2) electrode-pair, for the embodiment in FIGS. 7 and 8, for atotal heater power of 54 mW.

DETAILED DISCLOSURE

Embodiments of the subject invention relate to a gas sensor and methodfor sensing on or more gases specific embodiments pertain to apotentiometric gas sensor and method for sensing on or more gases.Additional embodiments are directed to amperometric and/or impedimetricgas sensors and method for sensing one or more gases. An embodimentincorporates an array of sensing electrodes maintained at similar ordifferent temperatures, such that the sensitivity and speciesselectivity of the device can be fine tuned between different pairs ofsensing electrodes. A specific embodiment pertains to a gas sensor arrayfor monitoring combustion exhausts and/or reaction byproducts. Anembodiment of the subject device related to this invention operates athigh temperatures and can withstand harsh chemical environments.

Embodiments of the device can have sensing electrodes in the sameenvironment, which allows the electrodes to be coplanar. The sensitivityand selectivity of these sensors can vary with temperature. Therefore,with respect to specific embodiments, the temperature of the device canbe precisely controlled and can be changed rapidly when desired. Inorder to achieve a device that is able to monitor two or more gasspecies of interest, an array of sensing electrodes can be incorporated.The array signals can then be entered into linear algorithms (or otherappropriate algorithm(s)) to determine the presence of and/orconcentrations of one or more individual species. As pattern recognitionis not an easy task to accomplish and may require additionalelectronics, thereby driving up the cost of the device, it may bepreferred to have the capability of individually monitoring a singlespecies in the presence of others, with minimal interference. In thisway, the device will not require extensive pattern recognition, if anyat all.

Embodiments of the invention can provide improvements in selectivity andsensitivity via thermal modification of individual sensing electrodesand/or the entire device. Furthermore, improvements in signal noise canbe achieved if the temperature is uniformly maintained. Also, with theincorporation of temperature control into embodiments of the subjectdevice, it is possible to reduce or reverse electrode “poisoning” orother phenomena that results in changes in sensor performance over timeor complete failure of the device.

The subject method and device can be used for the monitoring ofcombustion byproducts or other processes for chemical/gas monitoring. Ina specific embodiment, the device can be used to monitor the exhausts inautomobiles to determine if the catalytic converter has malfunctioned orto provide information for adjusting the air-to-fuel ratio in the enginebased on EPA (or other) requirements, which will change as drivingconditions differ. The subject device may also be used to monitorcombustion byproducts (or other chemical/gas related processes) at apower plant or any industrial manufacturing processes.

An embodiment of a sensor array in accordance with the inventionincorporates an integrated Platinum heater and temperature sensorsfabricated for small size and low power-consumption. The array includestwo La₂CuO₄ electrodes and a Platinum reference electrode all on thesame side of a rectangular, tape-cast YSZ substrate. Platinum resistorelements are used as heaters and/or temperature sensors to control andmonitor the temperature of the sensing electrodes. Finite ElementModeling was used to predict temperature profiles within the array. Thearray was then designed to keep one La₂CuO₄ electrode hot with respectto the other two electrodes. The results of from this devicedemonstrated that a gas sensor array with sensing electrodes kept atdifferent temperatures can yield a device capable of selectivelydetermining NO and NO₂ concentrations. In additional embodiments, theselectivity of a sensor array can be enhanced through control of thelocal temperature of the sensing electrodes.

Control of the local temperature of the sensing electrodes can beimplemented by cooling in addition to or instead of heating. Passiveand/or active cooling techniques known in the art can be incorporatedwith the subject invention.

Sensing electrodes can be made from metals (e.g., Platinum),semiconductors (e.g. semiconducting oxides such as La₂CuO₄ or WO₃), orother material showing sensitivity to a gas. In general, any givensensing electrode material will have varying sensitivity and selectivityto different gas species depending on the temperature of the electrode.The degree to which sensitivity and/or selectivity that changes dependson the material, gas, and temperature. Each electrode may be part of oneor more “electrode-pairs”. This means that the measurable number ofsignals can be larger than the actual number of sensing electrodes.Specifically, the design of the sensor array can include (either asindividual devices or together in a single device) two different“electrode-pair” schemes. One scheme can use multiple materials at thesame time, which may be kept at the same and/or different temperature.The control of the temperature can be accomplished via heating and/orcooling techniques. A device may also incorporate multiple electrodes ofthe same material that are maintained at one or more differenttemperatures. Electrodes of the same material may be kept at the sametemperature, one or more other features of the electrodes, such asmicrostructure, size, or thickness, can be different for differentelectrodes. Accordingly, the gas sensor arrays may utilize one or moreof these schemes in a single device, depending on the application.

Gas sensors in accordance with the invention can incorporatespecifically designed heating elements to control the temperaturetopside of individual sensing electrodes. In an embodiment, the sensingelectrodes are on topside of, and the heating elements are on thebackside, of a substrate. In another embodiment, the sensing electrodesare on both sides of the substrate. The substrate can be, for example, aYSZ substrate or other electrolyte. The substrate may also be astructural support, such as Al₂O₃, with an electrolyte layer on top. Theheating elements can be made of any material, such as platinum, that hasthe thermal and chemical stability to not degrade with time in a harshenvironment. The heating elements can act as resistors and produce heatvia Joule heating, by passing electrical current through the heatingelements.

In accordance with various embodiments of the subject invention, avariety of electrolyte materials for the substrate can be used and avariety of materials can be used for the sensing electrode and anyheating elements can be used. Examples of suitable materials are taughtin U.S. Pat. No. 6,598,596, which is incorporated herein by reference inits entirety. The electrodes can be made from a variety of materials,including metals, and semiconductors. The semiconductor material ispreferably a metal oxide or a metal oxide compound. The terms “”metaloxide” and ““metal oxide compound” are used interchangeably herein tomean a compound having elemental metal combined with O2. Examples ofmetal oxides that are useful in the invention include SnO₂, TiO₂, TYPd5,MoO₃, ZnMoO₄ (ZM), WO₃, La₂CuO₄, and mixtures thereof. The semiconductormaterials can include a metal oxide. The metal oxide is preferably SnO₂,TiO₂, TYPd5, MoO₃, or ZnMoO₄, where TYPd5 is an acronym defined below.The acronym TYPd5 is used herein to represent a composite prepared byselecting TiO₂ (titania), Y₂O₃ (yttria) and Pd in a weight ratio ofapproximately 85:10:5.

The electrolyte is preferably an oxygen ion-conducting electrolyte. Theoxygen ion-conducting electrolyte can be based on ZrO₂, Bi₂O₃ or CeO₂.Preferred oxygen ion-conducting electrolytes are electrolyte mixtures,the mixtures generally including a base material, such as ZrO₂, Bi₂O₃ orCeO₂ and one or more dopants, such as calcia (CaO) and yttria (Y₂O₃)which can function as stabilizers, or some other suitable oxygenion-permeable material. For example, yttria stabilized zirconia (YSZ)electrolytes can be formed by mixing yttria and ZrO₂. Electrolytes thatconduct ionic species other than oxygen ions, e.g., halides, are wellknown in the art and also find utility in the invention for measuringhalogen-containing gas species. The choice of material for electrolytecan depend on the component in the gas mixture to be measured. Thus, tomeasure the concentration of an oxide component, for example, NO_(x),CO_(x) or SO_(x) the electrolyte is preferably an oxygen-ion conductingelectrolyte. Preferred oxygen ion-conducting electrolytes areelectrolyte mixtures based on zirconia (ZrO₂), bismuth oxide (Bi₂O₃),and ceria (CeO₂). Practical electrolyte mixtures generally include oneor more dopants, such as calcia (CaO) and yttria (Y₂O₃), or some othersuitable oxygen ion-permeable material.

A specific embodiment of a gas sensor array includes two LCO sensingelectrodes and two Platinum reference electrodes. The inner LCO and Ptelectrodes are heated, while the outer LCO and Pt electrodes remain nearthe ambient temperature. Furthermore, the potential difference betweenmultiple pairs of electrodes can be measured in order to providesignals. Since no two electrodes have the same combination of materialand operating temperature, there are a total of six distinct signalsthat can be measured by pairing the four electrodes. These signals canbe compared to help determine the gas concentrations in a mixture ofgases.

The temperature control of these devices can be important. Precisecontrol of temperature with minimal fluctuations can allow the device toproduce stable sensor signals. Therefore, thermal modeling can beperformed during the design phase to provide information regarding thetemperature profile in the device for different locations of the sensingelectrodes and the heating electrodes on the substrate of the array.

Platinum can be used for the fabrication of heating elements andtemperature sensors. Platinum is an industry standard forhigh-temperature resistance-temperature-devices (RTD) and as heatingelements in gas sensors because of durability and chemical and thermalstability. However, other materials may be used as heaters in thesubject devices.

Surface temperature measurements can be difficult and some of the bestmethods available include use of optical infrared sensors and RTDs.Below approximately 400° C. the resistance of Platinum has a lineardependence on temperature. However, above this temperature, further heatloss causes the linear model to deviate from experimental data, and analternative model is

R(T)=a(1+bT−cT ²)   (1)

where a, b, and c are empirical coefficients. After the data is fit tothe model, software can calculate the surface temperature during sensoroperation using the coefficients from (1) and resistance measurements ofthe Platinum elements.

Heating elements can be used not only to heat another object but alsosimultaneously as a temperature sensor. If the resistance of the heatercan accurately be determined (e.g., using a four-wire method), then thetemperature of the Platinum element can be calculated. Resistanceincreases as current is supplied to the heater because of Joule heating.This does not greatly affect the voltage or current measurements. Thatis to say, the measurements represent the actual current in the circuitand voltage drop across the heater. Therefore the calculated resistance,and hence temperature, of the heater represents the real value.

The heating element shape is important to the temperature distribution.In an embodiment, the temperature of the sensing electrode is uniform,or, if desired, nonuniform in a preferred manner. In an embodiment, theheating elements are C-shaped. Serpentine-patterned heaters can also beutilized. Spiral shaped heaters, or any other shaped heaters, can alsobe used. The heating elements can be controlled either by an appliedvoltage or current. The method of controlling the heating elementsutilized depends on the application. As an example, in an automobile,the automobile's battery can be the power source, such that the heatingelements would be voltage controlled.

In a specific embodiment, a YSZ substrate can have multiple sensingelectrodes on one side. Platinum (or other resistive material) elementsare on the opposite side of the YSZ substrate, aligned with theelectrodes. The sensing electrodes may also be oriented in a symmetricor nonsymmetrical fashion with respect to each other, and they may bestaggered. The Platinum (or other resistive material) elements need notbe used as heaters. The Platinum elements may be used as heaters and/ortemperature sensors. In another embodiment, the semiconductive elementscan be used for cooling of the electrodes via, for example,thermoelectric cooling. The cooling elements may also be made of anymaterial which allows cooling of specific regions in the device. Thethermal characteristics of the heating/cooling elements and/or surfacetemperature sensors can be improved with the use of insulating materialsintegrated into the device structure or by other specific shape ordesign change to the device that impacts the thermal properties of thedevice, such as empty volumes. The shape of the substrate can also vary.

FIG. 1 shows a specific embodiment of a device in accordance with thesubject invention, and FIG. 2 shows a cross-sectional view of the sameembodiment. The device includes two La₂CuO₄ electrodes with a Platinumelectrode in between, on a first side of a substrate, where thesubstrate is an electrolyte. A Platinum heater and two Platinumtemperature sensing elements can be positioned on the other side of thesubstrate. FIGS. 3-6 show the sensor response vs. concentrations of NOand NO₂ for two different electrode-pair combinations taken from thethree sensing electrodes. These results show that the device was able toproduce a signal that was mainly sensitive to NO₂ and a signal that wassensitive to both NO and NO₂. Thus, indirect detection of individualconcentrations of NO and NO₂ is possible via substraction.

FIG. 7 shows another specific embodiment of the subject invention, andFIG. 8 shows a cross-sectional view of the same embodiment. The deviceincludes two La₂CuO₄ electrodes and two Platinum electrodes,interdigitated with each other, on one side of a substrate. On the otherside of the substrate, incorporating an electrolyte, are four Platinumelectrodes, where the inner two are heaters and temperature sensors andthe outer two are temperature sensors. This arrangement allows the twoLCO electrodes to be maintained at different temperatures and the twoPlatinum sensing electrodes to be at different temperatures. If the twoheated electrodes are maintained at constant temperatures, this allowssix electrode-pair combinations for receiving signals. If the heatedelectrodes are designed to have more temperatures during operation, thenmore electrode-pair combinations can be created, with a specificelectrode at two different temperatures acting as two electrodes for thepurposes of providing output sensor signals. FIGS. 9-10 show sensorresponses for NO and NO₂ for the electrode-pair having the non-heatedLCO electrode and the heated Platinum electrode showing the results forincreasing temperature difference and absolute temperature of eachelectrode. The slopes of the plots from FIGS. 9-10 can be taken, whichrepresent the sensitivity (mV change in signal per decade change in gasconcentration), to make trend plots provided in FIGS. 11-16. Each curverepresents a different heater setpoint, which in turn represents adifferent temperature difference between the electrodes for the deviceshown in FIGS. 7-8. This was repeated for each of the six electrodesignals from the four sensing electrodes for the device shown in FIGS.7-8. In the trend plots the curve where |dT| is equal to zero is thecase where the heaters were not being operated.

With respect to the device shown in FIGS. 7-8, more sensor signals canbe measured than the number of sensing electrodes on the device itself.This is possible because some of the electrodes are at differenttemperatures. Furthermore, the device can have electrode-pairs that areselective to NO only and other electrode-pairs that are selective to NO₂only. Other embodiments can have electrode pairs that are selective toother gases such as CO and CO₂. In fact, for some of the heatersetpoints there were examples of the electrode-pairs switching theirsignal direction as they went either more positive or negative. Thisindicates that for a given electrode material or pair of materials, ifthe temperature is kept different between them, then the electrode-paircan be utilized in a way that results in it being sensitive orinsensitive to one or more gases.

Furthermore, the sensors can take advantage of both changes in absolutetemperature and differences in temperature between electrodes making upelectrode-pairs. Sensitivity to a given species typically is altered athigher temperatures. If two sensing electrodes are brought above thetemperature where they are no longer sensitive to one gas, but both arestill sensitive to another gas, then the signal will be selective.Additionally, it is possible that if the temperature of one of the twoelectrodes is further increased that the signal, which is now selective,will also benefit from an increase in sensitivity as the individualpotentials of the electrodes is further changed. This can be takenadvantage of based on how the sensing electrodes' sensitivity changeswith temperature and the specific gas species the electrodes are exposedto. In specific embodiments, pattern recognition is not used, therebyreducing device costs and improving sensor performance. The performanceis also improved because one is able to increase the sensitivity of someof the electrode-pairs using the same methods for achieving differencesin temperature between the electrodes. This can also be done by changesin microstructure and geometry of device.

The array of sensing electrodes used for various embodiments of theinvention can include several different sensing electrodes. A referenceor pseudo-reference electrode can be included, if desired. Inembodiments, each sensing electrode can be used to make up a “sensingelectrode-pair.” Furthermore, each sensing electrode can be used incombination with other sensing electrodes in the array to make upmultiple electrode-pairs. Different electrode configurations orproperties will change the way in which the sensor performs. This allowsspecific tailoring of the device to achieve the desired performance(e.g., sensitivity, selectivity, and response time) for specificapplications.

Depending on the specific design and/or application, the sensingelectrodes can be configured using the same or different electrodematerials, using the same and/or different microstructures, using thesame and/or different geometries (shape and thickness), and/or beingoperated at the same and/or different temperatures. The key is that thetwo electrodes to be used to create a sensing electrode-pair, when anelectrolyte is in contact with the two electrodes, should create avoltage potential across the sensing electrode-pair when exposed to agas species to be measured or to a mixture of gases which includes a gasspecies to be measured. By having the two electrodes have somecombination of different microstructures, different geometries (shapeand thickness), different materials, being at different temperatures,and/or any other alteration which causes the materials to differ in someway, the conditions to create a sensing electrode-pair can exist.

Temperature control of the sensing electrodes can be used to achieve thedesired performance. Depending on the sensing electrode-pair, theperformance of the measured signal can generally be modified via thermalmodification. Furthermore, the temperature is preferably kept fromchanging due to external sources (such as changes in the gas streamtemperature). Therefore, embodiments of the device can incorporate ameans to monitor the temperature of the sensing electrodes and a meansto change their temperatures when needed.

Heating elements can be utilized to modify the temperature of thesensing electrodes when needed. The heating elements can be on theopposite side of a substrate from the sensing electrodes, eachappropriately aligned with a specific sensing electrode. Heatingelements can be located on the same side of the substrate as the sensingelectrodes as well. Heating elements may also be embedded in or on theelectrolyte or support. Different heating element patterns can beimplemented (e.g., C-shaped, spiral, or serpentine patterns) in order toyield the ideal thermal distribution on the device. The heat can begenerated by Joule heating (Heat=Power*Time=Current²*Resistance*Time).The heating current may be voltage or current controlled and deliveredin pulses or in a constant manner. The heating current may be deliveredby simple current splitting or by individual (current or voltage) outputto the heating elements.

The temperature of the sensing electrodes can also be controlled viacooling, either in conjunction with heating or alone. In an embodiment,cooling can be accomplished using a method known as thermoelectriccooling, for example, using a solid-state heat pump. Cooling can also beaccomplished with the use of heat sinks. By changing the temperature onother areas of the device, a temperature under a sensing electrode mayalso be lowered. Other designs to accomplish cooling of specific regionsof the device are also possible.

Temperature monitoring can be accomplished by measuring the resistanceor other temperature related parameter of elements made of metal,semiconductor, or other material that cover an area under or near thesensing electrodes. Temperature sensors also may be embedded or layexposed on the surface. Multiple methods of temperature sensing arepossible including use of RTDs and thermocouples. Temperature sensorsmay act simultaneously as heating elements or may be stand aloneelements. Temperature sensors may act simultaneously as cooling orheating elements as well.

There are several different signals that may be monitored. Some of thevarious signals that can be monitored include the voltage of sensingelectrodes and/or voltage differences of sensing electrode-pairs.Multiplexing can be used to monitor the multiple voltage signals fromthe corresponding multiple sensing electrode-pairs. Resistance or otherparameter monitoring of temperature sensors can also be accomplished andcan also utilize multiplexing.

Various embodiments incorporate a detector for measuring an electricalcharacteristic with respect to the sensing electrode. One method ofdetection in the sensor array may be potentiometric. The array mayinclude other methods of detection such as conductimetric (orimpedancemetric), capacitve, or other methods for detecting gas species.This extension of the sensor array can be achieved monolithically or onseparate substrates connected to a common measurement system.

There are numerous techniques that can be employed in the manufacture ofembodiments of the subject devices. Multiple devices may be madesimultaneously and separated by various means after manufacture. Anycombination of the following techniques can be utilized. Multilayerfabrication, such as tape-casting, and/or screen-printing, can be used.Bottom-Up (additive) approach, such as direct-write methods (e.g., pump-or aerosol-based deposition), laser micromachining, and/or lasersintering, can be used. Multi-step (subtractive) approach, such asmicrofabrication using photolithography and other techniques used in thefabrication of microelectronics and microelectro-mechanical systems(MEMS), and/or electron-beam and laser-based subtractive fabrication,can be used. Wire attachment methods and metallization, such as metalsused for metallization or wire attachment must be able to withstandharsh environments. Wire bonding (e.g., Au or Pt wire), brazing, and/orother methods of wire attachment can be used. Different metallization(materials or otherwise) may exist in multiple layers and connected toeach other by vias that exist in between layers or on the outside of thedevice. Device packaging can be accomplished via standard or otherpackaging techniques. Designs of high-temperature (or any other)electronics and/or sensors may be used with this device. These may beincorporated into the sensor for a monolithic device or exist as a partof a hybrid system.

Embodiments

Embodiment 1. A gas sensor, comprising:

a sensing electrode in contact with an electrolyte, wherein theelectrode is exposed to an environment of interest;

a mechanism capable of altering the temperature of the sensingelectrode; and

a detector for measuring an electrical characteristic with respect tothe sensing electrode, wherein the measured electrical characteristicprovides information with respect to one or more gases in theenvironment of interest.

Embodiment 2. The gas sensor according to Embodiment 1, wherein thesensing electrode is disposed on a surface of a substrate, wherein thesubstrate comprises the electrolyte.

Embodiment 3. The gas sensor according to Embodiment 1, wherein thedetector measures an EMF between the sensing electrode and a reference.

Embodiment 4. The gas sensor according to Embodiment 1, wherein thedetector measures the impedance of the electrode.

Embodiment 5. The gas sensor according to Embodiment 1, wherein thedetector measures a current in the electrode.

Embodiment 6. The gas sensor according to Embodiment 3, wherein themeasured EMF indicates whether a first gas is present in the environmentof interest.

Embodiment 7. The gas sensor according to Embodiment 3, wherein themeasured EMF indicates a concentration of a first gas present in theenvironment of interest.

Embodiment 8. The gas sensor according to Embodiment 1, furthercomprising: at least one additional sensing electrode.

Embodiment 9. The gas sensor according to Embodiment 8, wherein one ofthe at least one additional electrode is the reference.

Embodiment 10. The gas sensor according to Embodiment 1, wherein themechanism capable of altering the temperature of the sensing electrodecomprises a heater.

Embodiment 11. The gas sensor according to Embodiment 10, wherein theheater is in thermal contact with the electrolyte.

Embodiment 12. The gas sensor according to Embodiment 10, wherein theheater is detached from the electrolyte and sensing electrode.

Embodiment 13. The gas sensor according to Embodiment 10, wherein theheater radiatively heats the sensing electrode.

Embodiment 14. The gas sensor according to Embodiment 10, wherein theheater conductively heats the sensing electrode.

Embodiment 15. The gas sensor according to Embodiment 1, wherein theheater comprises the sensing electrode and a current source for drivingthe electrode with a current.

Embodiment 16. The gas sensor according to Embodiment 1, wherein themechanism capable of altering the temperature of the sensing electrodeis capable of cooling the sensing electrode.

Embodiment 17. The gas sensor according to Embodiment 10, wherein theheater comprises a heating element, wherein when a heating current ispassed through the heating element the heating element produces heatthat heats the electrode.

Embodiment 18. The gas sensor according to Embodiment 3, furthercomprising at least one additional sensing electrode in contact with theelectrolyte, wherein the sensing electrode and the at least oneadditional sensing electrode form an array of sensing electrodes.

Embodiment 19. The gas sensor according to Embodiment 18, wherein thereference is one of the at least one additional sensing electrode,wherein the reference has a different shape than the sensing electrode.

Embodiment 20. The gas sensor according to Embodiment 18, wherein thereference is one of the at least one additional sensing electrode,wherein the reference is at a different temperature than the sensingelectrode.

Embodiment 21. The gas sensor according to Embodiment 18, wherein thereference is one of the at least one additional sensing electrode,wherein the reference is made of a different material than the sensingelectrode.

Embodiment 22. The gas sensor according to Embodiment 18, wherein thereference is one of the at least one additional sensing electrode,wherein the reference comprises a different microstructure than thesensing electrode.

Embodiment 23. The gas sensor according to Embodiment 18, wherein uponexposure to a gas to be measured, an EMF occurs between a selected twoelectrodes of the array of electrodes.

Embodiment 24. The gas sensor according to Embodiment 18, wherein thearray of electrodes comprises electrodes formed of only the samematerial, wherein electrodes of the array of electrodes are maintainedat one or more different temperatures by the corresponding array ofheating elements.

Embodiment 25. The gas sensor according to Embodiment 24, wherein anytwo electrodes of the array of electrodes maintained at a differenttemperature function as an electrode-pair.

Embodiment 26. The gas sensor according to Embodiment 24, wherein theelectrodes formed of the same material and maintained at a sametemperature comprise one or more electrodes having differentmicrostructures, sizes, or thicknesses.

Embodiment 27. The gas sensor according to Embodiment 26, wherein anytwo electrodes of the array of electrodes maintained at a differenttemperature, having a different microstructure, having a different size,and/or having a different thickness function as an electrode-pair.

Embodiment 28. The gas sensor according to Embodiment 18, wherein thearray of electrodes comprises one or more electrodes of a first materialand one or more electrodes of a second material, wherein the electrodesof the array of electrodes are maintained at one or more differenttemperatures by an array of heating elements.

Embodiment 29. The gas sensor according to Embodiment 28, wherein anytwo electrodes of the array of electrodes formed of a different materialand/or maintained at a different temperature function as anelectrode-pair.

Embodiment 30. The gas sensor according to Embodiment 28, wherein theelectrodes formed of a same material and maintained at a sametemperature comprise one or more electrodes having differentmicrostructures, sizes, or thicknesses.

Embodiment 31. The gas sensor according to Embodiment 30, wherein anytwo electrodes of the array of electrodes formed of a differentmaterial, maintained at a different temperature, having a differentmicrostructure, having a different size, and/or having a differentthickness function as an electrode-pair.

Embodiment 32. The gas sensor according to Embodiment 18, whereinelectrodes of the array of electrodes comprise metal or a semiconductingoxide.

Embodiment 33. The gas sensor according to Embodiment 18, whereinelectrodes of the array of electrodes comprise at least one platinumelectrode.

Embodiment 34. The gas sensor according to Embodiment 18, whereinelectrodes of the array of electrodes comprise at least one La₂CuO₄(LCO) electrode.

Embodiment 35. The gas sensor according to Embodiment 28, wherein thearray of heating elements comprise resistor elements.

Embodiment 36. The gas sensor according to Embodiment 35, wherein theresistor elements are formed of platinum.

Embodiment 37. The gas sensor according to Embodiment 35, wherein eachresistor element is disposed in a pattern on the opposite surface of theelectrolyte to one of the electrodes of the array of electrodes.

Embodiment 38. The gas sensor according to Embodiment 35, wherein thepattern of each resistor element comprises a C-shape pattern, a spiralpattern, or a serpentine pattern.

Embodiment 39. The gas sensor according to Embodiment 1, furthercomprising:

a temperature sensor for measuring the temperature of the sensingelectrode.

Embodiment 40. The gas sensor according to Embodiment 1, wherein thesensing electrode is made of a semiconductor.

Embodiment 41. The gas sensor according to Embodiment 1, wherein thesensing electrode is made of a metal.

Embodiment 42. The gas sensor according to Embodiment 1, wherein thesemiconductor is a metal oxide or metal oxide compound.

Embodiment 43. The gas sensor according to Embodiment 42, wherein thesemiconductor comprises one or more of the following: SnO2, TiO2, TYPd5,MoO 3, ZnMoO4 (ZM) and WO3 and WR3, La2CuO4, and mixtures thereof.

Embodiment 44. The gas sensor according to Embodiment 1, wherein theelectrolyte is an oxygen ion-conducting electrolyte.

Embodiment 45. The gas sensor according to Embodiment 44, wherein theelectrolyte is based on ZrO2, Bi2O3 or CeO2.

Embodiment 46. The gas sensor according to Embodiment 1, wherein the oneor more gases are one or more NOx, CO_(x), and SO_(x).

Embodiment 47. The gas sensor according to Embodiment 1, wherein the oneor more gases is NO.

Embodiment 48. The gas sensor according to Embodiment 1, wherein the oneor more gases is NO₂.

Embodiment 49. The gas sensor according to Embodiment 1, wherein the oneor more gases are NO and NO₂.

Embodiment 50. The gas sensor according to Embodiment 18, wherein afirst electrode-pair of the array of electrodes provides a firstelectrical characteristic providing information with respect to a firstof the one or more gases and a second electrode-pair of the array ofelectrodes provides a second electrical characteristic providinginformation with respect to a second of the one or more gases.

Embodiment 51. The gas sensor according to Embodiment 50, wherein thefirst gas is NO and the second gas is NO₂.

Embodiment 52. The gas sensor according to Embodiment 18, wherein afirst electrode-pair of the array of electrodes provides a firstelectrical characteristic providing information with respect to a firstof the one or more gases and a second electrode-pair of the array ofelectrodes provides a second electrical characteristic providinginformation with respect to the first and a second of the one or moregases.

Embodiment 53. The gas sensor according to Embodiment 50, wherein thefirst gas is NO₂ and the two gases are NO and NO₂.

Embodiment 54. The gas sensor according to Embodiment 53, wherein theinformation with respect to the NO and NO₂ is the sum of theconcentration of NO and NO₂.

Embodiment 55. The method of sensing one or more gases, comprising:

exposing a sensing electrode to an environment of interest, wherein thesensing electrode is in contact with an electrolyte;

altering the temperature of the sensing electrode; and

measuring an electrical characteristic with respect to the sensingelectrode, wherein the measured electrical characteristic providesinformation with respect to one or more gases in the environment ofinterest.

All patents, patent applications, provisional applications, andpublications referred to or cited herein are incorporated by referencein their entirety, including all figures and tables, to the extent theyare not inconsistent with the explicit teachings of this specification.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication.

What is claimed is:
 1. A gas sensor, comprising: a sensing electrode incontact with an electrolyte, wherein the electrode is exposed to anenvironment of interest; a mechanism capable of altering the temperatureof the sensing electrode; and a detector for measuring an electricalcharacteristic with respect to the sensing electrode, wherein themeasured electrical characteristic provides information with respect toone or more gases in the environment of interest.
 2. The gas sensoraccording to claim 1, wherein the sensing electrode is disposed on asurface of a substrate, wherein the substrate comprises the electrolyte.3. The gas sensor according to claim 1, wherein the detector measures anEMF between the sensing electrode and a reference.
 4. The gas sensoraccording to claim 1, wherein the detector measures the impedance of theelectrode.
 5. The gas sensor according to claim 1, wherein the detectormeasures a current in the electrode.
 6. The gas sensor according toclaim 3, wherein the measured EMF indicates whether a first gas ispresent in the environment of interest.
 7. The gas sensor according toclaim 3, wherein the measured EMF indicates a concentration of a firstgas present in the environment of interest.
 8. The gas sensor accordingto claim 1, further comprising: at least one additional sensingelectrode.
 9. The gas sensor according to claim 8, wherein one of the atleast one additional electrode is the reference.
 10. The gas sensoraccording to claim 1, wherein the mechanism capable of altering thetemperature of the sensing electrode comprises a heater.
 11. The gassensor according to claim 10, wherein the heater is in thermal contactwith the electrolyte.
 12. The gas sensor according to claim 10, whereinthe heater is detached from the electrolyte and sensing electrode. 13.The gas sensor according to claim 10, wherein the heater radiativelyheats the sensing electrode.
 14. The gas sensor according to claim 10,wherein the heater conductively heats the sensing electrode.
 15. The gassensor according to claim 1, wherein the heater comprises the sensingelectrode and a current source for driving the electrode with a current.16. The gas sensor according to claim 1, wherein the mechanism capableof altering the temperature of the sensing electrode is capable ofcooling the sensing electrode.
 17. The gas sensor according to claim 10,wherein the heater comprises a heating element, wherein when a heatingcurrent is passed through the heating element the heating elementproduces heat that heats the electrode.
 18. The gas sensor according toclaim 3, further comprising at least one additional sensing electrode incontact with the electrolyte, wherein the sensing electrode and the atleast one additional sensing electrode form an array of sensingelectrodes.
 19. The gas sensor according to claim 18, wherein thereference is one of the at least one additional sensing electrode,wherein the reference has a different shape than the sensing electrode.20. A method of sensing one or more gases, comprising: exposing asensing electrode to an environment of interest, wherein the sensingelectrode is in contact with an electrolyte; altering the temperature ofthe sensing electrode; and measuring an electrical characteristic withrespect to the sensing electrode, wherein the measured electricalcharacteristic provides information with respect to one or more gases inthe environment of interest.